(1) Field of the Invention
The present invention relates to a method for producing compound semiconductor single crystals and a method for producing compound semiconductor devices, and further a method for heat-treating compound semiconductor materials. More particularly, the present invention relates to the technique available for producing Fe-doped InP single crystals and the compound semiconductor devices employing these single crystals as their substrates.
(2)Description of the Prior Art
Various methods for producing compound semiconductor single crystals have been well known. For example, a seed crystal is immersed in melted solution of the crystal in a crucible and then the seed crystal is pulled up with rotating it relative to the crucible to grow the single crystal from the seed crystal. As an alternative method, the melted solution is gradually solidified to grow the single crystal. In industrial scale, Liquid Encapsulated Czochralski Method (LEC Method) belonged to the former method, and Gradient Freeze Method (GF Method), Horizontal Bridgeman Method (HB Method), and Vertical Bridgeman Method (VB Method) belonged to the latter method.
Although these single crystal growing methods have a little difference with each other, crystals are grown in basically similar process that the temperature gradient between the crystal and the melted solution is generated to gradually solidify the melted solution. In the process, while the interface between the liquid and the solid where the crystal is grown is kept at the melting point, but the part to the crystal grown already is kept at the lower temperature than the melting point. Accordingly, these single crystal producing methods can not avoid the production of inhomogeneous single crystals.
For example, in order to produce MESFETs employing an undoped semi-insulating GaAs single crystals as the substrates, Rumsby provided a method for annealing GaAs single crystal ingots at high temperatures from 700 to 1000.degree. C. to decrease the variation in the electrical properties of the crystal. This ingot annealing method intended to transfer impurities and inherent defects such as EL2 in the crystal so as to homogenize the crystal properties, referring to "D. Rumsby, R. M. Ware, B. Smith, M. Tyjberg, M. R. Brozel and E. J. Foulkes GaAs IC Symposium, Phoenix, Technical Digest (1983)34-37".
Conventionally, Fe-doped InP single crystals have been also sometimes subjected to the ingot-annealing at the high temperatures when the single crystals have been used as the substrates for MISFETs or OEICs.
In order to fabricate electronic devices such as MESFETs, MISFETs, and so on employing the compound semiconductor, impurities such as Si, S, Se, or the like are ion implantated into the wafers and then heat-treated to form active layers, and electrodes and insulating layers have been deposited on the active layers. Conventionally, in the case that Fe-doped InP single crystals are used as the substrates, the heat-treatment to form the active layers has been carried out at 700.degree. C. or higher.
However, the heat-treatment of the crystal ingots at temperatures ranging from 700.degree. C. to 1000.degree. C. can homogenize the resistivity across the wafer to a certain extent, but this was not sufficient for practical use.
Therefore the electronic devices such as MISFETs and OEICs employing Fe-doped InP single crystals as the substrates can be used for research purposes, but they have not yet been practically used in industrial field. The practical use of these devices has been tremendously retarded in comparison with MESFETs using GaAs substrate. Even though some reasons for the above described demerits have been considered, the inventors have found that while the heat-treatment of Fe-doped InP single crystal ingots or wafers can homogenize the resistivity across the wafers after crystal ingot annealing to some extent, the heat treatment for activating the wafers at 700.degree. C. or higher deteriorates the homogeneity of electrical properties across the wafer. Even if the compound semiconductor devices are produced by employing thus produced wafers, such devices would not be satisfactory for the practical use.